When I first met my wife, she was constantly stressed out about something.  What I found works are a few things:
- occasionally buy her something small that she likes.  I used to buy her a single rose about once a week. It made her happy.
- I got very good at giving back rubs.  She said it really helped her deal with the stress.
- be there when she needs you and listen to her problems.  One thing I've learned along the way is that women like to vent and complain and they just want someone to understand where they're coming from, not necessarily fix something.  Men, conversely hear the complaining and start offering solutions.  If it seems appropriate, offer to help fix something, but otherwise just listen to her.

Anyway, that's worked for me.

Well I try to give her massages, and back rubs. And I always listen to her problems. But when she gets stressed she gets really snappy. And will starting picking at me and being really critical. So it becomes really hard to support her when she is stressed when she is always having a go at me when she is like this.

My wife did that a lot while we were dating, Leritzi.  It took many years, but my extreme sarcasm eventually broke the from her.  My suggestion is to tell her (when she's not in this mood, mind you) that what she does really bothers you and that you're going to have to do something different because you don't like it when she does that.  Tell her you want to be supportive, but she can't treat you like that.  Ask her if she has suggestions on what you can do.  She'll either come to terms with her behavior, she'll suggest something useful - like leaving for a while, or she won't respect you enough to want to change. 

Fortunately, my wife turned out to be pretty resiliant because when she did that to me, I'd just stare at her and not say anything until she got mad at me, or I'd tell her that she sounds just like her mom or something else I knew would push her buttons.  A lot of it is that she wanted to pick a fight.  I found it was best to fight on my terms about nonsensical things.